In electrophotographic apparatuses and electrostatic recording apparatuses, electric or magnetic latent images have been developed into images by the use of toner. For example, in an electrophotographic process, an electrostatic image or latent image is formed on a photoconductor, and then the latent image is developed by using a toner to form a toner image. Typically, the toner image is transferred onto a transfer material such as paper and then fixed by means of, for example, heating.
A toner typically includes a binder resin in an amount of 70% by mass or more. Since most of the binder resins are made from oil resources, there are concerns of depletion of the oil resources and the issue of global warming caused by discharge of a carbon dioxide gas into the air due to heavy consumption of the oil resources. If a binder resin can be synthesized from a plant which grows by utilizing carbon dioxide gas in the air, the carbon dioxide gas can be circulated. Namely, there is a possibility of preventing the global warming and the depletion of the oil resources. Therefore, polymers derived from plant resources (i.e., biomass) are receiving attention recently.
In attempting to use polymers derived from plant resources as a binder resin, a toner including polylactic acid as a binder resin is disclosed (see PTL 1). Polylactic acid is a commonly-used, easily-available polymer formed from plant resources as raw materials. It is known that polylactic acid is synthesized through dehydration condensation of lactic acid monomers or through ring-opening polymerization of cyclic lactides of lactic acids (see PTLs 2 and 3). However, when polylactic acid is directly used alone for the production of a toner, it is difficult to obtain necessary properties for a toner. This is because the concentration of an ester group is higher than that of a polyester resin, and the molecular chains bonded together via the ester bond are formed only of carbon atoms.
In one possible measure to overcome this problem, polylactic acid and a second resin different therefrom are mixed together or copolymerized to thereby ensure physical properties and thermal characteristics required for toner. For example, there has been proposed that a terpene-phenol copolymer is incorporated as a low-molecular-weight ingredient into a polylactic acid biodegradable resin for improving thermal characteristics (see PTL 4). This proposal, however, does not achieve both desired low-temperature fixing property and desired hot offset resistance at the same time, and the polylactic acid resin has not been practically used for toner. Furthermore, polylactic acid is quite poor in compatibility and dispersibility with polyester resins and/or styrene-acryl copolymers commonly used for toner. Thus, when such polylactic acid is used in combination with other resins, it is considerably difficult to control the composition of the uppermost surface responsible for important properties of toner such as storageability, chargeability and flowability.
As an example of attempting to solve the above existing problem through copolymerization, there has been reported a block copolymer resin formed between polyester resins other than polylactic acids and defined in D/L ratio of polylactic acids (see PTL 5). However, the strength of the binder resin formed from polylactic acids with this method is not necessarily high. As elucidated from studies conducted by the present inventors, the toner, the binder resin of which has low strength, causes background smear and scattering accompanied by stress applied during long-term stirring when used in the developing process.
In general, a binder resin for toner is designed to provide a toner with suitable chargeability and fixability as well as is required to have strength. When a rein having low strength is used, the produced toner is cracked or chipped by contact stress in the developing process. Toner dust formed as a result of chipping is easier to make the inside low-melting-point wax to be exposed thereon. Since such toner dust has large electrostatic or non-electrostatic attachment force onto a carrier, it remains on the carrier to cause toner filming. The carrier contaminated by toner filming decreases in ability to charge toner particles. As a result, so-called background smear occurs where toner particles are attached (printed) on blank portions. Similarly, as has been known, when the amount of charges which the toner can receive from the carrier decreases, the ability to electrostatically retain the toner on the carrier surface also decreases, so that the toner is scattered in the developing device during stirring to contaminate the developing device (i.e., toner scattering). The above-described problem similarly arises for the binder resin formed from polylactic acid. At present, satisfactory results have not yet been attained for improvement in durability of toner against stress applied during long-term stirring.